Image storage device



Dec. 24, 1957 E. E. SHELDN 2,817,78

IMAGE STORAGE DEVICE Filed May 27. 1954 s sheets-sheet 1 Bl/f/WW Dec. 24, 1957 E, E. sHELDoN IMAGE: STORAGE DEVICE Filed May 27. 1954 3 Sheets-Sheet 3 Alai/7 [05 INVEJVTOR.v fon/M0 EAW/aa 5,451 om hired States @are Patented nec. 24, les? iMAGE STGRAGE DEVHCE Edward Emanuel Sheldon, New York, N. Y.

Application May 27, 1954, Serial No. 432,748

tiaims. (Cl. 313-65) following U, S. Patents: No. 2,555,423, filed April 16, y

1947, and issued June 5, 1951; No. 2,699,511, iiled May 4, 1951 and issued January 11, 1955 and No. 2,717,971,

tiled March 30, 1949 and issued September 13, 1955.

One primary object of the present invention is to provide a method and device to produce intensified :i

images. This intensification will enable to overcome the inefliciency of the present iiuoroscopic examinations. At the present level of illumination of the uoroscopic image the human eye has to rely exclusively on scotopic (dark adaptation) vision, which is characterized by a tremendous loss of normal visual acuity in reference both to detail and to the contrast.

Another object of this invention is to make it possible to prolong the fluoroscopic examination since it will reduce markedly the strength of radiation alfecting the patients body. Conversely, the exposure time or energy necessary for the radiography may be reduced.

Another object is to provide a method and device to store X-ray images which was not possible until now.

The present intensifying devices concerned with reproduction of Xray images are completely unsatisfactory, because at low levels of tiuorescent illumination, such as We are dealing with, there is not enough of X-ray photons to be absorbed by tiuorescent or photoelectric screens used in such devices. Therefore the original X-ray image can be reproduced by them only with a considerable loss of information. it is well known that the lack of sutiicient number of X-ray quanta cannot be remedied by the increase of intensity of X-ray radiation, as it will result in damage to the patients body. This basic deliciencyof the X-ray examination was overcome in my invention by using an X-ray exposure of a strong intensity but of a short duration, and storing the invisible X-ray image for subsequent inspection of the desired length of time without any need of maintaining the X-ray irradiation. The X-ray beam, therefore, can be shut o while reading the stored X-ray image and in this way the total X-ray exposure received by the patient is not-increased, in spite of using bursts of a great X-ray intensity.

In order to obtain the objects of this invention a special X-ray sensitive image tube had to be designed, Fig. 1. This novel X-ray image tube is characterized by elimination of the optical lens system present in other image tubes which resulted in lO- fold gain in the light reaching the photo-cathode. Then by the combined use of a novel photoemissive pick-up system and storage system of the tube, of an electron multiplier section of the tube, of a 2 novel electron image amplifier section of the electronic acceleration and of the electronic image diminution the intensification of the luminosity of the original image exceeding the ratio of G-l was accomplished.

The elimination of the optical system present in other image tubes to focus the fluorescent image on the photocathode of the tube was accomplished by positioning Within 'the X-ray sensitive image tube of the screen, consisting of combination of X-ray transparent, light reecting layer, of X-ray fluorescent, or reactive layer, and of the photoemissive layer. All layers are placed in close apposition to each other to prevent the loss of definition. The fluorescent and photoemissive layers are separated only by a very thin iight transparent, chemically inactive, barrier layer of a thickness not exceeding 0.15 millimeter. the previous combinations of uorescent and photoemissive layers were not successful because of detrimental chemical interaction of both iayers, due to lack of a barrier between them, therefore the introduction of light f transparent barrier layer represents a very important part of this invention. The photoemissive layer is of semitransparent type. This layer is characterized by emission of electrons on the side opposite to the side of the in* cident light. The photoelectrons emitted from the photoemissive layer in a pattern corresponding to the incident light pattern are accelerated and focused by means of magnetic and/or electric fields on the novel image amplifying system.

The amplification section of the tube consists of one or a few screens each of them composed of a very thin liglit-reiiecting, electron pervious layer, of a uorescent layer, and of a photoemissive layer in close apposition to each other. lt is necessary to include a very thin light transparent, chemically inactive barrier layer between the Fluorescent and photoemissive layers in order to prevent their chemical interaction, which should be of a thickness not exceeding 0.15 millimeter but preferably much less. The electrons from the pick-up section of the image tube are focused by magnetic or electrostatic elds on the iiuorescent layer of a screen described above. The luminescence of the fluorescent layer of the amplification screen will cause the emission of electrons from the phetoemissive layer of the screen. This process can be repeated a few times, using a few screens described above resulting in 10-100 times intensification of the original electron image.

ln another modication to be used in this invention there is an additional multiplier section which consists of a multiplier and can give an additional intensication of the electron image by secondary electron emission.

The electrons leaving the amplifying section are accelerated by means of high voltage electro-static fields. The accelerating system can be of a conventional type Well known in the art. Much better results with higher voltages will be achieved with an electrostatic multi-lens system. Next the electron image is stored in the special storage target. The storage of the electron image allows the inspection of the X-ray image for a desired time without the need of maintaining X-ray irradiation during the reading. r[his saves so much energy in X-ray exposures that the patients body will not be impaired even with prolonged examination.

Next the electron image is demagnified which results in its additional intensification. The electron diminution of the image, in order to gain its intensilication is well known in the art, therefore does not have to be described in detail.

The diminished electron image is projected on the fluorescent screen at the end of the tube where it can be viewed by the observer directly or by means of an optical magnifying eye piece, through the light transparent end wall of the tube.

as meer Ille `combination of the above described .features of the X-ray sensitive image tube allows to obtain intensification of the original X-ray image which was the primary objective fof .this invention. Having such a marked intensification *of the original X-ray image it will be lpossible nowto use 'amuoh finer :grain of .fluorescent screens than was practical until now and to improve this way detail and :contrast of the final image, which was another purpose of this invention.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawings by way of example only, preferred embodiments of the inventive idea.

IIn the drawings:

fFig. 1 is ia cross-sectional view of the X-ray image storageitube.

Fig. 2 is `a cross-sectional view of a modification of the X-ray image storage tube.

Fig. .3 .is across-sectional view of the X-ray storage image tube in=combination with optical system.

Fig. i4 lis a cross-sectional view of a modification of the X-.ray image .storage tube.

F.ig. 5 is 'a cross-sectional view of a modification of the .X-ra-y imagestorage tube.

yFigs.16 an'da show a plan view of the storage target.

Fig. 7 is a diagrammatic View of the neutron sensitive photocathodefto be used in neutron storage tube.

Figs. Sand 9 show a modification ofan X-ray or a neutron-sensitive photocathode.

f'Fig. :10 isa cross-sectional view of a modification of the X-ray image storage tube.

Fig. lfl shows a modificationfof the storage tube using electrostatic focusingsystern.

Fig. .121shows a modification of the storage tube using an imperforatedistorage target.

Fig. 113 shows a storage tube adopted for transmission of images.

Fig. 14 shows a modification of the storage tube with animperforated target.

Figs. =15 and 1'6 shows a tube having photocathode of 'l photoconductive type.

Fig. 17 shows a kmodification of photoconductive photocell.

The face 1 ofthe image tube 40 shown in Fig. 1 must be of a material transparent -to ythe type -of radiation to be used. vInside of the face of the tube there is a very thin visible 'light reflecting, X-ray transparent layer Z such as of aluminum or light diffusing layer such as of titanium oxide which prevents the loss of light from the adjacent fluorescent layer 3. An extremely thin barrier layer y4 separates fthe fiuorescent screen 3 from the ad` jacent tphotoemissive vlayer 5. The reflecting layer 2., the uorescent layer 3, the separating layer 4, land photoemissive layer 5 form together a composite photocathode 5a 'which Lconverts X-ray images into electron images. In some cases .the composite photocathode must be of convex shapeinstead of fiat type, as will be explained below. The fluorescent 'layer 3 and the photoemissive layers -5 should .be ycorrelated so that under the influence of the .particular radiation yused there is yobtained a maximum output of photoemission. More particularly the fiuorescent .layer should .be composed, of a material having its greatest vsensitivity to the type of radiation to be used, and the photoemissive material likewise should have its maximum sensitivity to the wave length emitted by the fluorescent layer. Fluorescent substances that lmay be used are willemite, or other zinc silicates, BaPbSO4, zinc selenides, yzinc sulphides or calcium tungstate, lwith or without activators. The satisfactory photoemissive materials for the composite photocathode will be caesium oxide, y-caesium oxide activated by silver, caesium with antimony, caesium with telluriunn bismuth 'or arsenic or antimony, with `lithium or potassium. The barrier `layer 4 I.between the tiuore'scent and Aphotoemissive l'surfaces Vcan be an exceedingly thin transparent film-ofglass, -of Zul-i12,

4 of silicon, tin oxide, tin chloride or of a suitable plastic such as polyesters, e. g. Mylar made by DuPont Company in U. S. A., or of a conducting material such as known in the trade under the name Nesa made by Pittsburgh Plate Glass Company.

In order to prevent chemical interaction between the photoemissive and fluorescent layer, I found that light transparent layer must be of a thickness not less than 0.5 microns and preferably 'of l micron. It is also desirable to provide in some cases alight transparent layer which is electrically conducting and which may serve as a conducting base for the proper functioning of the photoemissive layer 5. I found that conducting layers loose their transparency when they are made thicker than 0.1 micron. On the other hand the light transparent separating layer if made of dielectric material such as silica or Mylar remains transparent regardless of its thickness. Therefore in some embodiments of the invention the separating layer is composed of two layers, one of dielectric material 4a which provides the protective barrier and the other layer 4b of conducting material which helps the function of the photoemissive layer 5.

The thickness ofthe separating layer should not exceed 0.15 millimeter, but preferably should be less than 25 microns to preserve definition of images. The light transparent separating layer 4 in the photocathode 5a may be deposited on the fluorescent layer 3 so that it doesnt require lany support by the walls of the tube. In modification of the composite photocathode the light transparent layer 4 such as of Mylar may be attached to the walls of the tube by means of metallic rings `and may provide then support for other layers. AIn another preferred vmodification the liuorescent layer 3 is embedded in the plastic such Vas Mylar which has a great tensile strength. The fiuorescent layer embedded in Mylar is self-supporting and can serve also as a support for other layers of the composite photocathode. Mylar can be dissolved in a proper-solvent. Therefore it may be made of any dcsired thickness so that the top of Mylar layer outside of the fiuorescent layer which will now serve as a light trailsparent separating layer may be made of a thickness -of the order of microns as it is necessary for the best resolu'tion of the reproduced image.

The use `of plastic embedded fiuorescent layer has a great advantage over the glass embedded fiuorescent layer because `the thickness of the plastic outside of the fluorescent layer can be controlled up to microns which cannot be done when using glass. This construction is especially suitable for l`the `composite screen used as a second stageas `shown in Fig. l. It may be added that Mylar will withstand processing of the vacuum tube.

The electron yimage obtained in the pick-up section is now accelerated by electrodes -6 and is transferred to the first screen of the amplifying section by means of focusing magnetic or electrostatic fields which -are not indicated, since vthey are well Aknown in the art and `would only .serve to complicate the illustration. The amplifying section .uses vone or a few successively arranged composite screens 5a :each of them consisting of an electron per-viens, light-reflecting .layer 8, of a liuoresccnt layer li', of light transparent 'barrier layer 1b, and of photoernissve layer 11. Fluorescent substances 'that may be used are willemite `or Vother zinc silicates, zinc selenides. zinc sulphide or Vcalcium tungstate with or without activators. The satisfactory photoemissive materials will be caesium oxide, caesium oxide activated by silver, caesium with antimony, `with bismuth or arsenic or antimony with lithium or potassium. The barrier layer 10 between the fluorescent and `photoemissive surfaces was described above and maybe of the same type. The amplification achieved by this system results in marked intensification of theoriginal image.

`In some applications it may be .preferable to use in conjunction with amplifying lsystem the electron multiplier section consisting of one or a few stages of secamarsi ondary electron multipliers which serves to intensify fur ther the electronic image. In such a case the electron image from the pick-up section of the tube is focused by means of magnetic field on the first stage of the multiplier section. The secondary electrons from the first stage are focused the same way on the second stage of the multiplier section and so on. CsOzCs or Ag:Mg multipliers provide a good secondary electron emission.

The electrons emerging from the amplifying section are now accelerated and imaged by means of elds 47 to the desired velocity, giving thus further intensification of the electron image. Next the electron image is diminished by means of electromagnetic or electrostatic lenses to the desired size, resulting in image intensification proportional to the square power of the linear diminution and is projected on the storage target.

The use of a storage target improves markedly signal to noise ratio resulting in pictures of much better detail and contrast. The storage target is shown in Fig. 6 and consists of a thin perforated metal sheet or of woven conducting wire mesh 41a. On the side of the target opposite to the photocathode there is deposited by evaporation storage material such as CaF2, 41h in such a manner that openings 41C in the target should not be occluded. in some cases on the side of the target facing the photocathode there is deposited by evaporation a thin metal coating.

`Between the photocathode and the storage target, in a close spacing to the target, there is mounted a ine mesh conducting screen 42. On the side of the storage target, opposite to the photocathode there is disposed a meshed metal electrode 43, which repels electrons during the writing phase of operation and attracts electrons during the reading phase. Adjacent to said meshed metallic screen there is disposed a fluorescent screen 44 provided with a metallic light reflecting electron transparent layer 44a such as ot aluminum. The reflector electrode 43 during writing is kept at the potential negative to the photocathode a. Therefore the photoelectrons transmitted through the perforated target are repelled by said retiector electrode and have to fall back on the storage target 41 and deposit thereon varying charges at s-uccessive points according to the pattern of X-ray image. The best way of operating my system is to have the storage target surface at zero potential or at photocathode potential and then to write on it positive which means to deposit positive charges. This can be accomplished by adjusting the potential of the surface of the storage target so that its secondary emission is greater than unity.

The photoelectrons having the pattern of X-ray image after passage through openings in the target are repulsed back by lthe reector electrode 43 because in this phase of operation its potential is lower than that of the storage targe The impingement of photoelectron beam causes secondary electron `emission from the target 41 greater lthan unity. The secondary electrons are drawn away. As a result a positive charge image is formed on the perforated target 41 having the pattern of the original X-ray image. This charge image can be stored in the target as .ong as 50 hours, if `the storage material is CaFg.

In the reading phase of operation of my device the target #ill is scanned by a slow electron beam 50 from the electron gun S2. The electron beam is focused by magnetic or electrostatic fields 49 and is decelerated by 'the electrode 42 which may be in the form of a ring or of meshed screen. The defiecting fields and synchronizing circuits are not shown in order not to complicate the drawings. It is obvious that all fields controlling the scanning beam are inoperative during the Iwriting phase of the operation. In the same way the elds controlling the photoelectron beam are not operating during the reading phase of the operation. A part of the scanning electrom beam 50a passes through the perforations 41e in the target 41. The charge image on the target controls the passage of the scanning electron beam 50a acting in the similar manner to a grid in the electronI tube.

yThe electron beam 50 in the reading phase of operation passes through the openings in the target 41 is modulated by the stored charges on it and strikes the iiuorescent screen 44- producing thereby a light image having the pattern of the original X-ray image. The fluorescent screen 44 is provided with an electron transparent, light-reflecting backing 44a such as of aluminum. A-t the same time the open mesh metallic screen 43 may be used as a collector for electrons to be converted into video signals and transmitted to distant receivers. In case no transmission of stored image is desired the image reproduced on the lluorescent screen 44 can be markedly intensified by using instead of the scanning electron beam a broad electron beam from the electron gun 52 covering all storage target 41 of a at ribbon scanning beam covering one line of the image.

Video signals can be obtained not only from the transmitted electrons of the scanning beam but as well from the electrons Sub of said scanning beam returning to the electron gun. This part of the electron beam is also modulated by the charge image on the target 41 but is of reverse polarity than the transmitted electrons. non-transmitted part of the scanning electron beam returns to the multiplier section 46. They are multiplied there and then are converted into video signals. This arrangement by using multiplication of electrons allows a marked intensification of video signals. The returning electron beam Sil/'J contains two groups of electrons. One group are electrons which are reected specularly from the target. Another one are electrons which are reflected non-specularly it means were scattered. These two groups can be separated from each other before reaching the multiplier. There are many ways to separate these two groups of electrons, all well known in the art. The best method is to introduce an additional helical motion into a primary scanning beam. Then the scattered electrons in the returning beam will be on one side of the specularly reflected electrons. Therefore it will be possible to direct scattered electrons into aperture of the multiplier, while stopping the reflected electrons by the edge of the multiplier aperture. The use of the scattered electrons increases markedly sensitivity of the system because it reduces the inherent shot noise of the scanning electron beam.

Video signals have the pattern of the original X-ray image. They are amplified and transmitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopes, facsimile receivers, in combination with electrographic cameras and others may be used to reproduce images for inspection or recording. The accelerating, focusing, and deflecting fields as well as synchronizing circuits are not known as they are well known in the art and would only complicate drawings.

After the stored image has been read and no further storage is desired it may be erased by the use of the scanning electron beam 50 by adjusting the potential of the storage target to the value at which the secondary electron emission of its storing surface is below unity. In such a case the target will charge negatively to the potential of the electron gun cathode. The potential of the reflector in the erasing phase of operation must be more negative than that of the storage target, so that the scanning electron beam will be repelled to the target.

lt should be understood that instead of composite photocathode 5a a single layer photocathode 5b of a material emitting electrons when excited by X-rays such as of lead, gold, bismuth or uranium may be used. This modification is shown in Fig. 2. rlhe operation of the X-ray image tube 7i) is similar to the tube 40 described above. The only difference being that in this tube the photoelectron image having the pattern `of the X-ray image is deposited as a positive charge image on the storing surface 41b l7 O'f tbsstoresltarsstfasins Vthe phvmcathode- The 111.19melectronLimagemay be deposited vron .the opposite side yof the target, .as well, if .a reector velectrode of .mesh 43 isprovided for this `purpose as was explained above.

In .another .modification .of my invention shown in Fig. 3 :the X-ray timage vis converted into a uorescent image 32a in .the fluorescent screen 31 outside of the X-ray mage storage .tube .33. The fiuorescent image is projected by au optical system, preferably of a reflective type 34 on the X-ray storage `tube `having a photocathode 35 of a material emitting electrons -such as of CsSb, CsOAg, or ofslithium cir-potassium on antimony or bismuth. The fluoreseentlimage .projectedon the Vphotocathode produces aphotoelectron image. The rest .of the operation of this X-ray storage tubefistthe same as described above.

'It should be understood v.that the storage surface 41b may bedepositedronthe side of the target 41 'facing the photocathodepronthe `opposite side of said target in all my remhodiments of invention. It is also to be understood that incase in which the storage surface 41b is facing the ,photocathoda the reflector electrode 43 should beeliminated as it'serves then no purpose and causes only deterioration-of the image.

-It -should 'be understood that the composite photocathode, -the electron gun and the perforated target may lne-disposed in many different ways and it is to be understood .that-the various modifications of their mutual arrangementacome Within the scope and spirit of my invention. l.One zof such modifications is shown by way of cxnmpleonly in Fig. .4. The X-ray image tube 53 operates `vin :the same `way .as vthe tube 70, the only difference being vthe photoelectron image is projected on the storage targetat an'angle which requires the use of arcuate focusing fields.

.Another modification of 4X-ray storage tube in which thephotocathode.and1electron gun are disposed at the oppositeends of the tube .is shown in Fig. 5. This arrangement is suitable .only for converting the stored X- ray .images-into video signals andcannot be used for immediate'reproduction of X-ray images in the same tube. ln this :modification 'of my invention, shown in Fig. 5, the :invisible fX-ray image of the examined object 32 is ccnverted'by `thecomposite photocathode 5a which 'has been -describedzabove into a .photoelectron image. The photoelectron image is accelerated by the electrode 47 and y.is focused by;` the magnetic or electrostatic fields 48 on thezperforated.storageztarget 64, also described above.

Between .thephotocathodeand the storage target, in a closesspacing totheitarget, Lthere is mounted a line mesh conductingiscreen 65. Onpthe side of the storage target, opposite lto thcphotocathode there is disposed a meshed metal.electrode 67. `The Vreflector electrode 67 during writing vtis kept Vat the lpotential negative to the photocathode '5a. Therefore :the photoelectrons transmitted through the perforated target are repelled by said reiiector electrode `and-haveto fall back on the storage target .64 and .deposit thereonrvarying :charges Vat successive points accordingto-'the patternof the X-ray image. The photoelectron :image may he also stored on the side of the storagetargettfacingthe'.photocathode as was explained above. In: such -a case thel storing surface should face the photocathode'and the meshelectrode 67 is eliminated, as itds-notmecessary. .The-.best'way of operating my system is .to have .the storage target surface at zero potential or at photocathode-potential. and then to write on it positive it means to .deposit positive charges. This can be accomplishedbyaadjusting the potential of the surface of the storageitargetso thatits-secondary emission is greater thaniunity.

,The photoelectrons 'having the pattern .of X-ray image after passa-ge through'openings intlietarget are repulsed backbythe reflector Velectrode 67 because in .this phase dfoperlation its'potential islower than that of the storage target. 'The impingement of photoelectron beam causes :gettarsiA secondary .electron emission from the target .64 -greater than unity.. The secondary yelectrons are drawn away by the mesh screen .64a of the Astorage target which is connectedto the source of .positive potential. As a result a positive .charge image is formed on the perforated target 64 having the pattern of the original X-ray image. This charge image can be stored in the target as long as 50 hours, vif .the storage material is CaF2.

.In `the reading yphase of operation of my system the target -64 is scanned by a slow electron beam 6 2 from the electron gun 72. The electron beam is focused by magnetic or electrostatic fields 71 and is decelerated by the electrode 73 which may be in the form of a ring or of meshed screen. The deflecting fields and synchronizing circuit are not shown in order not to complicate rthe drawing. It is obvious vthat all fields controlling the scanning beam are inoperative during the writing phase of operation. In the same way the fields controlling the photoelectron beam are not operating during the reading phase of the operation. A part of the scanning electron beam passes through the perforations in the target-64- The charge image on the target controls the passage of the `scanning electron beam 62 acting in the similar mannerto a grid in the electron tube.

lt is obvious that all fields controlling the scanning beam are inoperative during the Writing phase of the operation. In .the same way the elds controlling the photoelectron beam .are not operating during the reading-.phase of the operation. The scanning electron beam 62 is modulated by the stored charge image. A part of it, 162g, is transmitted through the openings in the storage target V64, is collected by the mesh electrode 65 and is rconverted into video signals in the usual manner. Another part, 6219, of the scanning electron beam is returning to the electron gun 72, is diverted to the multiplier 6?, and after multiplication therein is converted into video signals. Video signals have the pattern of the `original X-ray image and are amplified and transmitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopes, fascimile receivers, in combination with electrographic camera, and others may be used to reproduce images for inspection or may be photographed or recorded.

In some cases it may be more desirable to have the fiuorescent screen 44 mounted outside of the vacuum tube, in such cases thin electron transparent layer of chromium or aluminum is placed on the end wall 22 of the vacuum tube made of fernico glass. The image appearing on the iiuorescent screen can be viewed directly or by means of an optical eye-piece giving the desired optical magnification of the image. In other cases the fluorescent screen 44 is substituted by electrophotographic layer or by photographic layer in combination with fiuorescent screen, or by an electrographic plate, permitting thus to obtain apermanent record of electron image.

In another alternative of this invention the X-ray image tube is curved and the electron beam is defiected by proper magnetic or electrostatic fields. This arrangement will prevent the positive ions from reaching the photoemissive section.

It is not intended to restrict vthe scope of this invention to the employment of X-ray or gamma radiations but other corpuscular radiations and suitable reactive layers are intended to be comprehended. Another form of the invention is illustrated in Fig. 7, wherein a neutron reactive layer 26 preferably from the group boron, lithium, gadolinium land uranium or of parafiine is placed withinthe image tube to act as a photocathode. The protons or lelectrons yliberated from this layer 26 under therirnpact of neutron radiation .will strike directly or in some cases through an additional `thin electron ervious .chemically ,inactive Abarrier layer 4a', ,a suitable fluorescent layer 27, causing it to iluoresce and activate a suitable photoemissive layer 29 through the light transparent barrier layer 28. In other cases a neutron reactive layer of cadmium or copper will be more advantageous, because of its gamma emission. In some cases when the definition of reproduced images is not critical, the tluorescent layer 27 may be placed outside of the tube on its face 1.

In some cases it may be more desirable (Fig. S) to eliminate the fluorescent layer 27 and to cause protons or electrons from the layer 26a to act on electron emissive layer 29a either by apposition, or through an electron pervious chemically inactive barrier layer 30 which may be sometimes used either to prevent chemical interaction of adjacent layers or to slow down atomic particles from the layer 26a. In other cases it is preferable to focus protons or electrons or alpha particles from the layer 26a on electron emissive layer 29a with magnetic or electrostatic elds (Fig. 9). It should be understood that all modiiications apply to X-ray or gamma images as well. In case of X-ray images layer 26a should be preferably of gold, lead or bismuth.

The iluorescent layer to be used in the neutron sensitive tube may be of similar composition as described above in the X-ray sensitive image tube, but it has also to be adapted to respond most eiiiciently to the radiation emitted from neutron sensitive layer by enriching it with proper neutron reactive elements such as boron or gadolinium. The photoemissive layer has again to be correlated with spectral emission of fluorescent layer. The amplifying system, the multiplier system, the storage system, the electronic diminution are the same for neutron sensitive image tube and for X-ray sensitive image tube.

In another modification of my invention shown in Fig. l the X-ray sensitive pick-up tube 76 has a composite storage target 79 shown in Fig. 6a. The X-ray image is converted in the photocathode a described above into photoelectron beam having the pattern of the X-ray image. The photoelectron image is accelerated and focused on the perforated storage target 79. The target consists of a thin perforated light transparent dielectric such as glass 83. Also a metallic mesh screen can be used instead of a perforated glass. In such a case however, a light transparent dielectric must be deposited on the mesh screen and in such a manner that openings in the screen remain unobstructed. On the side of the glass layer 83 facing the photocathode is deposited a iluorescent layer 8l also in such a manner that the openings in the glass are unobstructed. On the side of the uorescent layer is deposited a light reecting layer 3d such as of aluminum. On the side of the dielectric layer 83 which is away from the photocathode is deposited a photoemissive layer 84 in such a manner that openings in the dielectric layer are not obstructed.

The photoelectron beam causes fluorescence of the layer $1. The fluorescent light passes through glass layer S3 and causes emission of electrons from the photoemissive layer 84. rI`he emitted photoelectrons are led away by adjacent collecting mesh screen 85. As a result a positive charge image is stored -on the layer 84. This stored charge controls the passage of the scanning electron beam 87 in the same manner as was described above. The transmitted electrons of the scanning beam will strike the uorescent screen 89 having a light reflecting electron transparent backing 8S and will reproduce therein the X-ray image. The transmitted electrons are focused on the fluorescent screen 89 by means of magnetic or electrostatic iields which are well known in the art.

It should be understood that the storage tubes shown in Figs. l, 3, 4, 7, S, l0, 1l, 12, 13 and 14 may be markedly simplified by eliminating the electron gun 52 and by using as a reading electron beam, the electron beam produced by the photoemissive layer 5 or- 35. This may be accomplished by irradiating the photoemissive layer 5 or 35 with a source of light 101 from outside in the image reading phase of operation.

Storage of X-ray images may also be accomplished by using a light image feed-back system. The fluorescent X-ray image in this modification of my invention is reproduced on the face of the image tube as was explained above. Before it begins to fade it is projected on a television pick-up tube and produces an electrical image therein. The electrical image is converted by the pick-up tube into video signals in the manner well known in television. For the purposes of this invention any type of television pick-up tube such as photoemissive type, photoconductive type, or of photovoltaic type may be used. Video signals are sent from the pick-up tube to the kinescope and reproduce there the lluorescent image. The fluorescent image from the kinescope may be again projected o-n the pick-up tube to produce again an image therein. In this way an endless stream of fluorescent light images is produced so that the uorescent image may be inspected for a desired time without maintaining the X- ray exposure.

It should be understood that all my storage tubes may be of the electrostatic or of magnetic type. In case electrostatic focusing means 48a are used the photocathode should have a slight curvature to improve detiuition of reproduced image as shown in Fig. 11. The same applies to all other types of photocathodes. In such a case the light reflecting layer 2 which may be of aluminum may serve also as a supporting member. The electron beam from the composite screen 5a, b, or 35 may be intensied `by the secondary electron emissive screen such as screen 2% shown in Fig. ll. I found however that velocities of secondary electrons emitted from the screen 2% vary considerably and they proceed in divergent directions. This leads to deterioration of denition of reproduced image. In my embodiment of the invention I eliminated this defect by using the secondary electron emissive screen 2917 in the shape of a lens. In this way I may provide suitable potentials so that divergent secondary electrons will be electron-optically focused.

It should be understood that all my storage tubes will be useful in producing images formed by radio isotopes whether they are present in the human body or used in industrial applications. The image formed by radio isotopes such as radio-iodine which emits penetrating gamma radiation cannot be focused on my image storage tube because there are no optical systems, which can focus gamma rays. As a result no image from radio-isotopes distributed in an organ could be obtained. I solved this problem by using in combination with my storage device a pinhole camera as illustrated in Fig. ll. The pinhole camera M7 is made oi' lead or other material which is able to stop penetrating gamma radiation. The camerali?? is provided with a pinhole opening w3. The storage tube of any type described above is introduced into said lead camera 107 and receives the image passing through the pinhole opening lltl'zt. In this way the gamma image may be now focused on the image sensitive screen of the storage tube. The use of a pinhole causes however a great decrease of radiation available for image formation. Therefore my storage devices are of great importance to make such system workable. The weak radio isotope image can be stored in the storage target d?. or and by a long exposure may build up a charge pattern on the storage target which is strong enough to be reproduced as a visible image.

Another embodiment of my invention is shown in Fig. 12. In this modification of my device the perforated storage target is replaced by a target having a continuous imperforated surface of dielectric material such as silica, titanium compounds, BaFz or CaFg. The photocathode may be of any one of the types shown and may be in the form of a composite screen 5a, as shown in Fig. l, or may be in the form of the composite screen as shown asi-arci in Fig-Y 7 `or 8. 0r in the form ef a screen 5b, .as shown in Fig. 2, or screen H3 5 as shown in Fig. 3 or 26a as shown in Fig. 9. The beam of electrons emitted from the photocathode 5a and having the pattern of the X- ray or other radiation image is accelerated and is focused by magnetic 48 or electrostatic means 48a on the novel storage target 90 which consists of a continuous jsheet 90 of a dielectric material such as described above. The impingment of said electron beam causes secondary emission of electrons from said dielectric material which may be higher or lower than unity according to the Velocity of the electron beam or potential applied to said storage target. The secondary electrons may be led away by the adjacent mesh screen 91. In some cases it may be preferable to have mesh screen 91 deposited on the surface 91a of the target 90 which is facing the composite screen 5a. It the electrons are of the velocity at which the secondary electron emission is higher than unity, a positive charge pattern having the pattern of the original image will (remain 'stored in the dielectric target 90. The electron beam from the photocathode could not pass through dielectric target 90 to its opposite side but the charge image can produ-ce by capacitance eiect a corresponding charge pattern on the opposite side 91b of dielectric target 99. On the side of the dielectric target 91 facing the electron gun 94 is disposed in a close spacing a mesh screen 92. The charge pattern appearing on the surface 91h sets up by capacitance effect a similar charge pattern on the mesh screen 92. Screen 92 is irradiated by a beam of electrons 93 from the electron gun 94. The beam 93 after the passage through the opening 95 in the diaphragm 96 is decelerated by decelerating electrodes 9'7 and is defocused to produce a broad beam 98. The broad electron beam 9S approaches the mesh screen 92 with a very low velocity and is modulated and reilected by the positive electrical pattern on said screen. The reflected broad electron beam has now therefore the imprint of said electrical pattern. The reilected modulated electron beam is striking the iluorescent screen 99 through the light reecting layer 99a such as aluminum and reproduces the visible image which is the replica of the original image. The uorescent layer 99 is supported by the light transparent diaphragm 96 which should be of preferably light transparent material such as silica or Mylar so that the observer may study the uoresce-nt image through window 102. The advantage of this novel embodiment lresides in the fact that the reading electron beam 98 does not discharge the stored charge image because it does not come in contact with it at all. Therefore the stored 'image may be read for a desired time without the necessity of maintaining the action of the invisible or visible image forming radiation. As a result my device will protect the living tissues from an excessive radiation. The stored charge image may be removed when desired by producing a broad electron beam of the opposite sign than the stored charge image. Such a broad beam of electrons may be produced by irradiating the photoemissive layer 5 which is inactive at this stage of operation with an extraneous source of light 101 or of ultra-violet. In some cases it may 'be preferable to provide for this purpose an additional electron gun 94a which will produce the erasing electron beam.

Another embodiment of my invention is shown in Fig. i3, which discloses the storage tube with a continuous imperforated target of the type illustrated in Fig. `12, and which may be used for transmission of images. The tube itl@ has defiecting means S to produce a scanning motion of the line-focus electron beam 193 across the mesh screen 92. The electron beam 103 after being reflected and modulated by the electrical pattern on the mesh screen returns to the multipliers 19,6 as beam 1035i. The multiplied electrons from the multiplier 195 are converted into .video signals overa suitable resistaucein a manner well Lknown in the v art.

Another 'mndifeaticn .0f ntyinventiotl is Shown in Fs- 14, jin which the mesh screen 92 is eliminated and 'the scanning electron beam '103 'is allowed to impinge on the target 90. 'This construction does not permit a long reading ofthe image as Idevices shown in Figs. 12 or 13 because the 'impingement of the electron beam on the target causes neutralization of the positive charges.

Anothermodication of myinvention is shown in Fig. 15 -whichlillustrates'the novel-image tube 108a. The invisibleirnage -passes-'throughtheface 1 of the tube, which obviously must be vof material-transparent to the radiation used andmay-be-ator convex in shape and strikes the compositephotocathode6z-disposed in the image tube. The composite photocathode 6a consists of an invisible radiation transparent,flight rellecting layer 2, of a uorescent layer 3 sensitive :tosaid invisible radiation, of a very thin conducting 4layer-4b and of a photoconductive layer 25. The layer 2 may ,be Vof aluminum, gold, silver or platinumand ymust -be very :thin in order not to absorb theinvisiblemage. The fluorescent and light transparent conducting layers rmay bethe same as described above.

For neutron images, `the iluorescent layer should be activated with elements .which Lhave a large cross-section for neutrons, such .as .-boron, lithium, gadolinium or an additional neutron sensitive layer, such as of boron, lithium .or -gadolinium, should be disposed adjacent to liuerescentlayenas wasexplainedabove. vIn some cases the fluorescent 'layer should be eliminated and the layer 26a may be appliedin contact with the photoconductive layer 2S as shown;` in Fig. 16.

The photcenductv layer 25 ,may .beof CdS, SbZ-TSS, selenium, ZnSe, or PbQ. Many sulphides, .antimonides, selenides, iodides, arsenides and oxides exhibit photoconductive Leffect and imay be `yu sed for .the purposes of -my inventicn. .The invisible image .produces yin the uorescent layer 3 a ,tluorescent light image having the pattern 0f .Said .invisible image- Ihe. -uQrescent image produces within the photoeonductive layer a pattern of changes in electrical `conductivity land on the surface of said photoconductive layer @pattern 0f potentials according to the pattern of `said `finerescent light image. The .photoconductive layer l25 is .under 1;he influence .cf a-n electrical field produced ,by anextrinsic source of electrical power, such as battery, 4which is connected to the conducting layer 4b. Under the iniluence of this electrical held, the electrons and positive holes liberated in the photoconductive layer by fthe impingement of fluorescent light from the layer .3 lmove lto respective electrodes. Therefore, the pattern .of potentials #having the pattern of the original X-.ray or ,neutron image appears on the uncovered surface of the photoconductive layer 25. In some cases, better results are .obtained .by usinga pulsating electrical field instead :of @batter-r 1H particular, applying a vSquare wave voltage ,of a low frequency, such as l5-L30 cycles per second ,to thegonducting layer 4b will markedly improvetheK-,sensitivityof thephotocathode and will prevent fatigue effects- The unCQVQIIQd Surtfac ,0f ,layer 25 is irradiated by a broad beam -93 Qf electrons from theelectrou gun 94- `The broad electron beam is fouSd by magnetic or electrostatic fields Ito asmall diameter, so, that it will pass through the-apertureS in thelight transparent diaphragm 9.6, such as of mica, ysilica or glass. The electron beam 93 after pas-sage through @PS YIUIC is enlarged by suitable magnetic or electro-static. fields Cto the size corresponding to the size of the photocathode .6a. The electron beam 93 when approaching v ph otocathode, may have velocity of a few hundred Volts. Itis necessary, however, to use a slow electron beam. Therefore the electron beam 93 is decelerated in front of the photocathode by an additional decelerating.electrode'Q/l, Lwhich may `be in the form of a ring or.of.a mesh screen. The .electron beam approaching the photoconduotive layer25 .is modulated ,by the pattern of potentials unitssurfacesrDie ,areas of a higher negative pQtential willnretlect .electrons more than areas haw.

. 13 ing a lower potential actingas an -electron mirror 11A. The Vreverse situation exists Aif the X-ray induced conductivity isfdue to positive holes, because in such a case, the areas of higher positive potential will obviously attract electrons instead of repelling them. The reliected electron beam 93 is therefore modulated by the potential im` age in the composite photocathode 6a and carries the replica of the original image of the examined body.

The retlectedelectron beam 93 irradiates the lluorescent screen 99, which has electron transparent light retlecting backing 99a, such as of aluminum. The impingement of reflected electron beam 93 on fluorescent screen 99 will reproduce the original invisible imageV as a uorescent light image. The lluorescent screen 99. should preferably have a very tine grain. ZnO or ZnSCdS phosphor is suitable for this purpose. Better denition will be obtained by evaporated phosphors which have no grain structure and are, therefore, capable of reproducing images of high definition. The lluorescent image can be viewed by the observer through the window 102 and through magnifying optical system. ln some cases, the light reflecting layer 99a may be omitted and image may be viewedffrom the uncovered side of the lluorescent screen. In such a case, however, photoconductive layer 25 and the uorescent screen 99 mustl be so correlated that a Wave length of the fluorescent light should not affect the photoconductive layer 25. For example, antimony trisulphide is not sensitive to blue ilght or selenium may be prepared not to be sensitive to thev red light.

A very important feature of my novel image tube is that it can be operated asa storage tube. This means that after the invisible image is formed in the photocathode 6a as aV pattern of electrical conductivity changes or of electrical potentials, the imageV forming radiation may be shut o l and the image may. be read for the desired time. This results in a great reduction of X-ray or neutron exposure of patients, which was one of the primary objectives of my invention. The operation of the image tube 108s as a storagetube is essentially the same as described above,y except that X-ray or neutron radiation may be stopped after one short exposure. The storage effect of my image tube is due to photoconductive lag observed in insulators, such as selenium, cadmium, sulphide or antimony trisulphide and others whenthe incident light is, of a low intensity. Such conditions prevail in medical tluoroscopy where` the brightness of fluorescent lightimage produced inlayer 3 b y X-ray or neutron image is in1 the range of` 0.G l0.00l footoandle. The photoconductive lag means that conductivity pattern within the layer 25 and potential pattern on the uncovered surface of4 said photoconductive layer persists for many seconds. During all this time, the electronbeam 93 can be modulated by said conductivity or potential pattern and will reproduce visible image coiresponding to the original X-ray or neutron image in theuorescent screen 99. The photoconduetive lagk may be prolonged by refrigerating the photoconductive layer 25 of the photocathode, or by addition ofV suitable impurities, Isuch as Cu when using CdS for a photoconductive layer. `It'should be added that the tube 108e shown inl Fig. l5 may be provided with a composite screen 5a for receiving an image and-converting said image :into a beam of. free electrons. The beam 0f frerel electrons will in turn act on the compositescreen 6a to produce a stored potential image as was explained above. l

-Another important advantage of my photoconductivc tube resides. in the eiciency of the photoconductive layer as ,compared with the previously used photoemisslve layer. Whereasthe best'photoemissive materials have quantum eiciency olf'theorder of-3- to 5%, the photoconductive layer 254 has quantum efficiency close to unity or even exceeding unity. lijhegeiiiciency ofrphotoconductive layer 2,5 canalsobeincreased by providing a strong electrical eld across it, which serves tomove liberated' electrons and positive holes across said'layer.

A great problemin quality of images produced by my devices is the reproduction of contrast values Which'rne'ans reproduction of half-tones. l found that I may improve the contrast and in addition amplify arbitrarily the contrast of reproduced images by using in iront of the photocathode of any type described above a mesh screen 24 as shown in Fig. i3. The mesh screen 24 is connected to the source of an extraneous electrical power such as battery and may be supplied with various necessary potentials. By providing the mesh screen 24 with a potential which will cause cut-olf of electrons of certain predetermined velocity we may vary arbitrarily the contrast of reproduced image. The mesh screen 2d has Wide mesh openings in order not to cause the loss of a large percentage of electron beam. It should be understood that the use or" contrast regulating mesh screen 2&5 is intended for all embodiments of my invention.

It should be understood that the image reproducing screen 4din all types of tubes described may be of material capable of the storage of the image. Screen 44 may be of persistent phosphors such as iluorides which have, a long after-glow. In some cases screen 4d may be of a scotophore material which means of material changing its color under the inlluence of electron irradiation. There are many scotophores known in the art such as halides; especially useful is potassium chloride. in case a scotophore type of screen is used means must be provided to return this screen to the original condition after the image has been read. ior this purpose we may use either heating of said screen or we may use a strong beam of electrons Which may be supplied by the photoemissive layer o the composite screen irradiated by a strong extraneous source of light. it should be understood that the image appearing in the reproducing screen of iluorescent type or of tenebrescent type or other types may be photographed or recorded on a suitable surface to provide a permanent record thereof.

It should be understood that all modifications of my device are suitable also for the storage of images produced by ultra-violet radiation. in such a case the photocathode may be of composite type such as shown in Pig. l. The uorescentlayer will be of material sensitive to particular wave-length of ultra-violet radiation used. For example, phosphorsy such as calcium phosphate, calcium silicate or barium silicate with suitable activators are sensitive to ultra-violet rays of 30G() A. lu case ultra-violet radiation is of a shorter wave-length such as G-3G00 A. the photocathode may be of a single layer type using potassium, sodium salicylate or silver chloride as a photoemissive layer.

My invention in all its modifications may be also used for the storage of infra-red images. 'in such case the photocathode may be of composite type as shown in Fig. l. The fluorescent layer may be of one of infra-red sensitive phosphors such as alkaline earth sulphides or selenides activated -by ceriurn, samarium or europium. ln case infrared radiation is of a longer Wave-length such as 2-2O microns we may use a single layer photocathode of material such as PbS, tellurides, antimonides or of barium titanate or other titanium compounds.

lt may bev added that the preferable way of dissolving Mylar is to heat it to temperature of 250 C. at which it becomes liquid. When it is in a liquid condition we may add toit a desired quantity of a phosphor. After a thorough mixing We let the solution cool oli and solidify again. lu this Way a self-supporting layer of fluorescent material anda light transparent separating layer of Mylar of' the desired thickness such as l-20 microns are produced. On this composite screen we may deposit a light transparent conducting layer and a photoelectric layer such as a photoemissive layer, asdescribed above.

In some cases the fluorescent layer and light transparent conductive layer may be eliminated and the neutron reactivelayeror the X-ray reactive layer 26a may bezap.- plied ,indirect contactl with the side of the photoconductive layer 25 which is facing the image. This arrangement is shown in Fig. 16 and is especially suited for very penetrating X-rays or gamma rays. The materials for neutron or X-ray reactive layer were described above.

In some cases the image tube shown in Fig. l may be simplied by using as a photocathode instead of a composite screen provided with a liuorescent layer, a screen of material such as cadmium, sulphide or lead oxide which when irradiated by X-rays or neutrons change their elec- 'trical conductivity. The remaining part of the tube may be the same as described above and shown in Fig. 15.

The layer of CdS or of PbO may be preferably deposited on a conducting layer of material transparent to the image forming radiation such as gold, platinum or tungsten. A source of electrical potential should be connected to said conducting layer to provide the bias potential.

It should be understood that all tubes described above may be adapted to transmit the images instead of reproducing them in the same tube. Such adaptation of one ot the embodiments of my invention to serve as a pick-uptube, which means a transmitting tube, will be shown in Fig. 17. The tube 109 is characterized by the photocathode 110 comprising a dielectric layer of material showing changes of electrical conductivity when irradiated by an invisible radiation and a conducting layer 112 transparent to the image forming radiation. Layer 111 may be of CdS, PbO, PbS or PbSe, Layer 112 may be of gold, platinum or silver. A source of direct current such as battery or preferably of a low frequency alternating current should preferably be connected to layer 112 to provide the biasing potential.

The X-ray or other invisible radiation is converted in the layer 112 into a pattern of electrical conductivity changes. The photocathode 119 is scanned by the slow electron beam 113 from the electron gun 94. The electron is decelerated in front of the photocathode 11i) by the decelerating electrode 114 to almost zero velocity.

The impingement of the scanning electron beam on the layer 111 produces electrical impulses at the conducting layer 112 which may be converted over a suitable resistance into video signals. The returning electron beam 113s is also modulated by the pattern of conductivity changes in the photocathode 110 and therefore carries also video information. The returning beam 113a may be directed to multipliers 115 and after electron multiplication may be converted into video signals. The focusing coils 116 and deeeting coils 117 are not illustrsted in detail as they are well known in the art.

Tube 199 may also serve as a storage tube by providing in front of the photocathode 110 a mesh screen ln which will serve as an electron mirror and will reflect the scanning electron beam 113. in the rst stage of operation. of the tube the scanning electron 113 will be allowed to pass through the mesh screen 111m by a suitable choice of potentials. The electron beam 113 will thus deposit electrons on the dielectric layer 111. After the layer 111 was subjected to the image forming radiation, the potential pattern will appear on the mesh screen l10n by a capacitance effect from the layer 111. This potential pattern will now serve to reflect and modulate the scanning electron beam 113. In this phase of operation the potential of the mesh screen is so adjusted so that the scanning electron 113 cannot pass through it but is reflected instead.

it should be understood that all embodiments of my invention can be further improved to make them able to discriminate between X-ray or gamma images and images produced by atomic particles. This is a very important feature of my invention as many sources of atomic particles are at the same time a source of gamma radiation. l obtained the differential response of my devices to one type of radiation only by providing all my modiflions of invention with a mesh screen of conduct ing material 24 such as shown in Fig. 13. The screen 24 should be of material not emitting secondary electrons. The mesh screen 24 is connected to the source of electrical potential. 'By applying proper potential to said screen 24, the electrons caused by impingement of gamma rays and which have a relatively low velocity will be cut-off by said mesh screen. -On the other hand the electrons or other secondary atomic particles which are produced by impingement of the primary image forming atomic particles and which are of relatively high `velocity will be able to pass through the mesh screen. In this way discrimination between gamma images and atomic particles image is accomplished.

It will thus be seen that there is provided a device in which the several objects of this invention are achieved and which is well adapted to meet the conditions of practical use.

As various possible embodiments might be made of the above invention, and as various changes might be made in the embodiment above set forth, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

l. A tube having in combination a photocathode com prising a continuous photoelectric layer for receiving an image and converting said image into an emitted beam of electrons, said photoelectric layer comprising an element of the group antimony and bismuth, an apertured member having a planar shape disposed in the path of said beam and transmitting particles of said beam emitted by said photocathode and a screen comprising an electron pervious light reflecting layer and luminescent means for receiving said transmitted electrons.

2. A device as defined in claim 1, in which said apertured member is a mesh screen of conducting material and is connected to a source of electrical potential, said source producing only a unidirectional current.

3. A vacuum tube having luminescent means and photoelectric means and light transparent separating means comprising a plastic material, all aforesaid means forming together a composite screen.

4. A vacuum tube having a composite screen comprising luminescent means and photoelectric means and light transparent separating means comprising a polyester` 5. A vacuum tube having in combination luminescent means for producing a luminescent image, a photocathode comprising a continuous photoelectric layer for receiving said luminescent image and converting said image into an emitted beam of electrons forming an electron image, said photoelectric layer comprising an element of the group antimony and bismuth, a mesh screen connected to a source of electrical potential disposed in the path of said beam and controlling transmission of electron of said beam emitted by said photocathode and a screen comprising an electron pervious light reflecting layer and second luminescent means producing a second luminescent image corresponding to said electron image.

References Cited in the le of this patent UNITED STATES PATENTS 2,060,977 De Boer et al Nov. 17, 1936 2,177,360 Busse Oct. 24, 1939 2,203,352 Goldmark June 4, 1940 2,219,113 Ploke Oct. 22, 1940 2,432,084 .Blair Dec. 9, 1947 2,532,644 Robinson Dec. 5, 1950 2,603,757 Sheldon July 15, 1952 2,726,328 Clogston Dec. 6, 1955 OTHER REFERENCES Fiat Final Report 1027, The Krawinliel Image-Storing Cathode Ray tube, April 2, 1947. 

4. A VACUUM TUBE HAVING A COMPOSITE SCREEN COMPRISING LUMINESCENT MEANS AND PHOTOELECTRIC MEANS AND LIGHT TRANSPARENT SEPARATING MEANS, COMPRISING A POLYESTER. 